Prepare dough: In a medium bowl, mix the water, salt and yeast. Stir in the flour until flour is completely incorporated. Cover the bowl and let the dough sit in a warm place for 1-2 hours until doubled. (Or leave on counter for up to 8 hours.)
When ready to bake, preheat oven to 475°F and cover a baking sheet with parchment paper.
Prepare filling: In a medium bowl, mix the tomatoes, Italian seasoning, garlic powder, onion powder, salt, and pepper.
Roll out dough into a large rectangle, about the length of the baking sheet.
Spread about 1/4 cup of the tomato sauce on the dough.
Sprinkle shredded cheese on top of the sauce, then top with the pepperoni.
Folding lengthwise, take the top 1/3 of the dough and bring it over to cover the center of the dough. Repeat with the other side, almost like you're folding a letter.
Place the stromboli, seam side down, on the prepared baking sheet.
Using a sharp knife, cut slits in top of dough, about every 1-2 inches. Brush the dough with the beaten egg and then sprinkle with 1 teaspoon of Italian seasoning.
Bake for 10 minutes.
Lower temperature to 375°F and bake for another 10-15 minutes until the dough is set and golden.
Let the stromboli sit for at least 10 minutes before slicing.